Hi. The current implementation of `future.utils.raise_from` refers to [PEP 3134](https://www.python.org/dev/peps/pep-3134/): https://github.com/PythonCharmers/python-future/blob/39a066ed/src/future/utils/__init__.py#L388 However, it does not set the `__traceback__` attribute as the PEP recommends. This would be very handy to display chained exceptions tracebacks. Would you be open to a PR adding this ?